(1) Field of the Invention
The invention relates to the detection and correction of errors in information processing and, more particularly, to detection and correction of errors in signals transmitted on a broadband network utilizing a three-level duobinary AM/PSK coding format.
(2) Prior Art
Error detecting and correcting techniques for processing information are well known in the prior art. Most error detecting and correcting techniques are directly related to the type of coding technique used to handle the data. U.S. Pat. No. 4,118,686 describes an isolated (single) error correction scheme for processing data that is coded with a modified duobinary code. The error detection technique uses the special properties associated with this specific code to identify and correct an error. Error correction is predicated on the concept of maximum likelihood of error for the bit having the maximum amplitude variation from the corresponding nominal amplitude level.
U.S. Pat. No. 3,418,631 described an error detection technique for information coded with a paired selected ternary (PST) code. In particular, the technique detects transmission errors that introduce a non-zero DC component. The non-zero DC component may cause a serious problem if accumulated over a tandem connection of many transmission channels which cannot transmit DC or low frequency components of a signal. Specific properties of the PST code are utilized to identify the unwanted condition. The detected error bits are replaced with a unique code having no DC component. The frequency of occurrence of this code is used to monitor the status of each link in tandem connection.
U.S. Pat. No. 4,276,649 describes a technique for minimizing the noise due to transmission errors in digitized voice systems. The technique is especially effective for delta modulation systems. This is done by detecting the error in the form of code violations in bipolar coded signals (alternate mark inversion code) and generating idle channel noise when error is detected.
U.S. Pat. No. 3,866,147 describes a technique for implementing a correlated ternary code. The code is claimed to have a power spectral density similar to a duobinary code but has more data transitions which can be used for clock recovery in the receiver. Error detection is based on the specific properties of the code.
Recently, the IEEE Local Area Network (Project 802.4) has proposed new standards for allowing a token passing multiple access scheme on a broadband network. The scheme uses duobinary AM/PSK modulation to achieve 5 Mb/sec data rate in a 6 MHz channel. A ternary signaling convention is proposed. Compared with a binary signaling format, ternary signaling format is more susceptible to noise. In terms of a bit error rate (BER) versus signal-to-noise ratio (SNR), ternary signaling requires 6 dB higher signal-to-noise ratio compared with binary signaling. The higher SNR requirement raises a problem in that the system is more vulnerable to transmission errors caused by noise in the broadband network. (Note that in broadband network, the signal power cannot be increased beyond a certain maximum power level without affecting other signals sharing the broadband network).